How do I get her to seek a diagnosis / help.
You mentioned she went to therapy for PTSD- is she still in therapy? If you approach her with BPD, chances are pretty good that it will go over horribly since she will feel like you're accusing her of being crazy. I've been here for about three years now and have not heard of a single instance where suggesting a diagnosis went well. For me personally, it was the last real conversation we had before our marriage was clearly over.
Maybe its the shame or guilt...I'm not sure...but we were more distant than ever after that conversation and there was no coming back from it.
You're much better off letting a professional have that conversation with her, and that could be a doctor, a therapist, or a psychiatrist. The diagnosis is often hard to make without family support and sharing, so it would be helpful for you to talk to her counselor IF INVITED. But taking it on yourself could have the opposite effect (in her mind).
She withheld it from me for 33 years and a few weeks before she left went to therapy for PTSD when the problem is BPD?
Complex PTSD and BPD are strikingly similar to the point where they often get mis-diagnosed. In general, someone with BPD experiences trauma as a child and learns inadequate coping skills to properly communicate in relationships (and that's parents, co workers, loved ones, etc). Complex PTSD is almost the same criteria and the same treatments, so I wouldn't get too hung up on which one it is. The important part is for her to get treatment and move past this.
Greater, Greater is he who dwells in me than dwells in the world! I never lost hope as I just did not understand as I took it personal due to my ignorance.
We were all ignorant because there's no possible way to identify BPD and react accordingly. So much of the communication style is counter-intuitive and it takes a lot of work to become proficient. Take a look at the tools section of this site (at the top of the page) to get started and familiarize yourself with some of the techniques.